1. Field of the Invention
This invention relates in general to electronically steered, two-dimensional, conformal, phased array antennae, and in particular to such antennae having a two-dimensional subsurface, traveling wave excitation. This invention is related to co-pending application U.S. Ser. No. 07/687/662, now U.S. Pat. No. 5,347,287, for a Conformal Phased Array Antenna, which describes an earlier embodiment of this invention.
2. Description of Related Art
Prior art in the field of electronically steered phased arrays, has mainly focused on electrically large two dimensional traveling wave arrays, with electronic beam steering in two planes and endfire beams. Such arrays are very densely populated, and include many hundreds, if not thousands, of elements. Further, in cylindrical configurations, wraparound conformal arrays physically extending 360 degrees around the cylinder axis, become possible in order to achieve at least a full hemispherical beam steering coverage of the top hemisphere, or an almost full spherical coverage. In airborn radar applications, wide off-airframe axis beam steering, close to the airframe roll plane, is actually easier to obtain from cylindrical arrays than endfire beams, as it corresponds to broadside radiation from most of the array elements. A two dimensional traveling wave array, radiating an endfire beam, planar or conformal, is somewhat equivalent to an array of Yagi-Uda arrays. Attaining such wide beam steering coverage makes many simultaneous conformal array operational functions possible, including high speed, wide volume radar target searches and multiple target tracking under severe terrain and sea clutter environments.
Examples of current phased array technology include U.S. Pat. No. 4,348,679 to Shnitkin et al, in which a single transmitter is used to generate electrical energy which is propagated through a waveguide to multiple power dividers to create branches similar to that of a corporate feed network. The novelty in Shnitkin is that an intermediate ladder configurations is used to form a front feed and a rear feed to provide excitation to the radiation elements. Each radiation element has its own feed line, resulting in a parallel configuration, which is complex, costly, and heavy. The range of beam steering in Shnitkin et al is limited to directions forward of the radiating elements, unlike this invention which, is capable of 360 degree steering because of its two-dimensional structure.
Lamberty et al, in U.S. Pat. No. 4,939,5277, disclose a distribution network for a space-fed phased array antenna comprising at least one orthogonal waveguide with a row of slots, one slot corresponding to each waveguide. The slots which provide the excitation wave feed into an electronics module which consists of a phase shifter and amplifier which are then connected to the radiating element. Each of the electronics modules is fed in parallel from the waveguide, as opposed to applicant's invention which teaches a series approach to feeding the elements with one phase shifter corresponding to each feed line so that it is associated with multiple antenna elements.
In U.S. Pat. No. 4,673,942 to Yokoyama, a multi-beam array antenna uses a matrix of feed lines, with one power feed line dedicated to each radiation element. The sole advantage of the Yokogama patent over the prior art is the introduction of delay lines in each power feed line to cause the excitation phase distribution to vary symmetrically around the center radiating element. The Yokoyama patent does not provide any simplification of the prior art by minimizing the number of feed lines within the feed network, nor does it provide for the feeding of more than one radiation element by a single feed line.
In co-pending application U.S. Ser. No. 07/687/662, a system was disclosed which includes a new feed network configuration that can be designed to physically fit within a very small internal depth below the external surface of an airframe, and to perform a load bearing structural function. A new method of array-excitation reduced the number of primary array feed lines and control elements, particularly when frequency scanning is used in one of the two beam steering planes. The broadband capabilities of tightly coupled delay structures reduce fabrication tolerance problems and make difficult broadband array applications more feasible. Finally, an optional active array architecture eliminated the need for combining transmit and receive functions in complex T/R modules, and for using one such module to feed every array element.
In the basic design underlying this co-pending invention, all the radiating elements of an electrically large, planar or conformal array antenna are mutually interconnected through a single, matrix-like, delay structure. The matrix-like delay structure extends behind the array aperture, and propagates guided waves in any direction parallel to the array antenna aperture surface. The delay structure is fed all around the array antenna aperture perimeter through a comparatively small number of peripheral input ports. The selected input ports form an excitation wave line source extending along a different segment of the array perimeter for different desired directions of the radiated beam. Electronic beam steering in a plane parallel to the array antenna aperture is obtained by controlling a small number of microwave solid state switches and phase shifters inserted along the array in external feeding lines. The switches first select the location of the set of active input ports along the array perimeter. The phase shifters then control the progressive phasing of the corresponding input signals. Because of the wave propagation properties of the underlying matrix-like delay structure, guided array-excitation waves are propagated in any desired direction parallel to the array aperture, and are dependent upon the settings of the switches and phase shifters. The radiated beam is then steered full circle in a continuous conical scan around the normal to the array aperture. Electronic beam steering in a plane orthogonal to the antenna array aperture is obtained either by frequency scanning or by electronically controlling the phase velocity of the guided array-excitation waves through the underlying delay structure. Either of these methods is physically equivalent to electronically controlling the Brewster incidence angle between the radiated beam and the guided array-excitation waves. Relatively broadband performance of electrically large planar or conformal arrays is obtained by designing the underlying matrix-like, delay structure as a tightly coupled cluster of multiport microwave resonators. Multiband performance is obtained by distributing different size array elements across the aperture in a regular pattern resulting from intermeshing at least two array lattices with different geometrical periodicity. Elements then are fed through mutually stacked independent delay structures. In an optional active architecture, two mutually stacked, matrix-like delay structures, both extending behind the antenna array aperture and having equal phase velocities, are interconnected at corresponding nodes by active, solid state amplifiers, in a two dimensional, distributed amplifier configuration. The upper delay structure is directly connected to the array antenna elements. Both delay structures perform, in turn, the functions of input and output circuit, depending on whether the array is in transmit or receive mode. Power amplifiers used in transmission are connected with the output ports towards the array elements. Low noise amplifiers used for reception are connected with the input ports towards the array elements. The two types of amplifiers are gated on and off in a mutually exclusive way.
In this underlying design, two simultaneous constraints have been implied in the choice of the relative amplitudes and of the relative phases of the microwave array-excitation signals, namely:
a) That all the external excitation signals have equal amplitudes, i.e. a `uniform` amplitude distribution along either set of external ports. PA1 b) That the relative phases of the microwave excitation signals injected through either set of external ports is represented by a step-wise linear progression of values, with a positive or negative constant phase difference between adjacent ports.
These tacitly implied assumptions are consistent with the simplest type of traveling-wave excitation of a two-dimensional clustered array, where a single pseudo-planar excitation wave is generated along one side of the aperture, and is made to travel across the array aperture as a single series of mutually-parallel, straight linear wavefronts oriented at some controllable angle, with respect to the rows and columns of the array elements.
With this type of traveling-wave array excitation, which is constrained by the above-formulated assumptions, electronic beam steering around the broadside direction i.e. in the direction of the equatorial angle, is obtained by controlling the direction of propagation of the traveling excitation waves. Electronic beam steering in a plane through the broadside direction in the direction of the polar angle, however, requires the electronic control of the wavelength of the excitation waves inside the cluster structure. Such control may be obtained by exploiting the cluster dispersivity by either tuning the operating frequency of the array, or by electronically tuning all the resonant array elements simultaneously, and by nominally the same amount.